In addition to rotating treatment tools there are also reciprocating treatment tools, i.e. treatment tools which carry out a stroke movement, for example having a flat or laminar cross-section. A treatment tool of this kind may be a tool for working on or cleaning teeth.
A handpiece head of this kind is described in U.S. Pat. No. 3,552,022. In this known construction the treatment tool can be clamped into a stroke sleeve that is mounted to be longitudinally displaceable in bearings in the handpiece head and can be displaced longitudinally by an eccentric of a drive shaft which in the present exemplary construction extends as an angular head at right angles to the stroke sleeve. The eccentric engages in a peripheral groove in the stroke sleeve. The stroke sleeve and the treatment tool can thus rotate freely even during operation and therefore with this construction it is only possible to work with the full face of the available, in particular laminar, longitudinally displaceable treatment tool, in particular a flat file. As soon as the person carrying out the treatment turns the handpiece slightly to exert torque on the treatment tool the latter also rotates and therefore specific forming or profiling work cannot be carried out on the tooth simply because the treatment tool always adjusts to lie flat against the surface to be treated.
If a drive connection were to be provided on the treatment tool with which the treatment tool is mounted so that it can move longitudinally but cannot rotate in the handpiece head, torque could indeed be exerted onto the treatment tool with the handpiece and thus specific forming work could be carried out on the tooth or between teeth, but there would then be the danger that with the aforementioned torque a considerable lever effect would be applied to the tooth which could damage it. An example of this would be if a flat file sitting between two teeth were rotated. The flat file would spread the teeth apart due to its rotation and it must be taken into consideration that, owing to the effective lever arm provided by the length of the handpiece, a considerable torque can be conveyed with the treatment tool through which damage could be caused to the tooth.
To avoid the difficulty described above it has been proposed according to WO/90/00885 with a handpiece head of the kind mentioned in the introduction, to limit the torque from the handpiece that can be conveyed by means of the treatment tool to the tooth. According to FIGS. 1, 1A and 2 of this publication an angular handpiece head is provided and the treatment tool has a securing shaft with which it can be inserted from the operating side of the handpiece head into a longitudinally reciprocating stroke sleeve in the handpiece head and in which the securing shaft is positioned in the inserted condition by a form-locking positioning coupling.
There are several positioning coupling positions available, in the form of a plurality of recesses in the lower edge of the stroke sleeve into which a coupling pin at the lower end of the securing shaft of the treatment tool can be introduced as desired by inserting it from below and from which the coupling pin can be withdrawn again by pulling it downwards. With the embodiment shown in FIG. 2 an excessive rotational force (over-turning) protection device in the form of an over-turning coupling (a coupling which releases or is overcome in response to excessive rotational force) is associated with this known handpiece head and comprises one or several axial and groove-shaped recesses distributed around the periphery, and a coupling member in the form of a ball that is arranged in a recess in the inner wall of the housing so that it projects into the interior and thus engages in one of the groove-shaped coupling recesses. The stroke sleeve consists of a plastics material having a certain flexibility or resilience. When torque is conveyed with the handpiece by means of the treatment tool onto the tooth to be treated that is greater than a specific torque, the rib walls between the coupling recesses yield so that the over-turning coupling formed in the manner described above is released or is overcome. Damage to the tooth is hereby intended to be avoided. This known embodiment is disadvantageous for several reasons. The torque by which the over-turning coupling is to be released or overcome cannot be determined exactly, so that different over-turning or release moments arise, depending on the consistency of the material, with which damage to the tooth already occurs. Furthermore it must be anticipated that after over-turning, altered resistance values of the material result, so that slackening of the over-turning coupling must be reckoned with. In addition the positioning coupling can only be operated with difficulty: the treatment tool must be grasped directly by hand, which impairs the sharpness of the treatment tool and is questionable on grounds of hygiene. According to FIGS. 3 to 18 the treatment tool is formed integrally with a securing- and drive shaft. With such a construction the positioning coupling and the over-turning coupling are thus directly effective between the housing and the treatment tool. As shown in FIG. 15 there are not only the coupling recesses of the positioning coupling arranged in the upper region of the securing shaft of the treatment tool but also a single coupling recess is provided in the lower region of the securing shaft in the manner of a longitudinal groove, so that the longitudinal displaceability of the treatment tool is ensured.